The present invention relates to the production of artificial fuel logs from a mixture of combustible material and binder. More particularly, this invention relates to the production of such logs by a method which includes compressing the mixture in a mold.
Artificial fuel logs, such as are suitable for burning in fireplaces, are well known. Such logs are made from a variety of combustible materials, although sawdust is the material most often used. Binders used include both thermoplastic and thermosetting types, with thermoplastic petroleum distillates such as wax being quite popular. Artificial fuel logs are made by both extrusion and compression methods.
An extrusion method for making artificial fuel logs is described in U.S. Pat. No. 4,042,343 of Bernard. According to this method, a mixture of sawdust and hot wax is conveyed to a cooler, where much of the heat from the wax is removed. The cooled mixture is then transported through a surge hopper to an extruder. The mixture is extruded through a die so as to assume a generally cylindrical, elongated log shape, and the log shape is cut into individual logs of the desired length.
Because extrusion methods for making artificial fuel logs are quite energy-intensive, the amount of energy produced by burning a log made by such a method may not be substantially greater than the amount of energy required to make the log. Another disadvantage of extrusion methods is the requirement that the combustible material used in the mixture of combustible material and binder be of uniformly small particle size to accommodate extrusion of the mixture through a die. It is to satisfy this requirement that fine sawdust is the material most often used in making logs by extrusion methods. Compression methods, however, offer the advantage of flexibility in the particle size and in the uniformity of particle size of the combustible materials used.
A compression method for making artificial fuel logs is described in U.S. Pat. No. 3,192,561 of Archer et al. According to this method, a quantity of tree bark chips is coated with paraffin wax and the wax-coated chips are placed in the bottom section of a split mold die of circular cross-section. The upper section of the die is then lowered from above onto the bottom section to compress the mixture of chips and paraffin radially into a cylindrical log shape. A cylindrical mold, which is shorter in length than the split mold but of comparable internal diameter, is then positioned in coaxial alignment with the split mold, and a ram is used to push the log shape out of the split mold and into the shorter mold. A second ram then cooperates with the first ram to compress the log shape in the shorter mold axially from both ends. The finished log is then ejected from the shorter mold.
Although they have been quite popular, artificial fuel logs made with binders of wax or other petroleum distillates have several undesirable features. Such logs typically derive the major portion of their fuel value from the binder. Consequently, they burn at higher temperatures and they produce less heat per unit of weight than do natural logs. Furthermore, when heated, such logs evolve hydrocarbon vapors which support combustion and which, if confined, may be explosive. It is for this reason that burning of such logs in airtight woodburning stoves is not advised.
Artificial fuel logs made with binders of wax or other petroleum distillates also exhibit low hot strength. Because of the relatively low melting point of the petroleum distillates used in such logs, they become pliable under relatively warm ambient conditions. Furthermore, they tend to lose cohesion very rapidly, and they often collapse almost immediately after being warmed throughout. For this reason, this type of log cannot be burned on andirons but must be burned on a grate.
Because of the disadvantages inherent in a use of thermoplastic binders, artificial fuel logs have also been made using non-thermoplastic binders. These logs are usually made by a compression method. For example, U.S. Pat. No. 3,986,845 of Hotchkiss describes a method for making such logs from chips of wood or cardboard, using as a binder a water-soluble glue. According to this method, chips of wood or shredded cardboard are charged into the upper end of an inclined, rotating drum. A water-soluble glue is sprayed on the chips as they enter the drum. The tumbling of the chips in the drum coats the chips with glue, and the propelling impetus of the glue spray helps to move the chips downwardly and out of the lower end of the drum. The coated chips emerge from the drum into a compacting chamber having a generally cylindrical cross-section, and periodically, a hydraulic ram is caused to move axially in the cylindrical chamber to compress the chips therein into a log shape. Finally, the ram is used to eject the finished log from the compacting chamber.
It is also known that artificial fuel logs may be made by a compression method using a thermosetting binder such as a urea formaldehyde resin. In my U.S. Pat. No. 4,220,453, I describe and claim a compression method for making artificial fuel logs in which such a binder is preferably used. According to this method, a mixture of combustible material and binder is prepared, and a quantity of the mixture sufficient to fill a tubular, open-ended mold is divided into two portions. One portion is then introduced through each of the open ends of the mold, and axial compressive forces are applied to the mixture in the mold from each end, so as to compress the mixture within the mold. The ends of the mold are then closed to maintain the mixture therein at a fixed volume, and the mixture is at least partially cured in the mold. The closures are then removed from the ends of the mold, and the finished log is removed from the mold.
The artificial fuel log made by the method of my U.S. Pat. No. 4,220,453, using a thermosetting binder, may be burned safely in a fireplace or in an airtight, woodburning stove. The amount of energy produced by burning such a log is substantially greater than the amount of energy required to make the log. In addition, the energy produced by burning such a log is derived almost entirely from the burning of the combustible material in the mixture from which the log is made. This log exhibits high hot strength, and when the combustible material used is a wood product, its combustion characteristics are quite similar to those of a natural log of comparable size. Furthermore, this log can be made using combustible materials of various types and of various particle sizes. However, it has been found that this log may vary somewhat in density along its length. Depending on the extent of the density variation, the log may not burn uniformly. Therefore, it would be advantageous if a log could be produced having all of the advantageous features of logs produced by my prior method, but also having a relatively uniform density along its length.
It has also been observed that some consumers prefer an artificial fuel log that is easier to ignite than a natural log of comparable size. Therefore, it would also be advantageous if a log could be produced which has all of the advantageous features of logs produced by my prior method, but which also may be ignited more easily than a natural log of comparable size.